1. Field of the Invention
The present invention relates to a process for producing organic carboxylic acid esters from the corresponding carboxylic acids and alcohols in a gas phase.
Not only are organic carboxylic acid esters useful compounds for coating compositions, adhesives, perfumes, plasticizers and solvents, but also, especially unsaturated carboxylic acid esters, are important intermediates as monomers or comonomers for various functional resins.
2. Description of the Related Art
The following processes have been known for producing organic carboxylic acid esters from the corresponding organic carboxylic acids and alcohols.
(1) Esterification in liquid phase using acid catalysts such as sulfuric acid, phosphoric acid, benzenesulfonic acid and p-toluenesulfonic acid.
(2) Continuous esterification in liquid phase using acidic cation exchange resins as catalysts.
(3) Esterification in gas phase using heteropoly-acid catalysts (Japanese Patent Kokai Nos. 57-99556 and 57-13095), composite oxide catalysts (Japanese Patent Kokoku Nos. 42-6324 and 45-24564 and Japanese Patent Kokai No. 51-75019), and catalysts comprising acids such as sulfuric acid, phosphoric acid, and benzenesulfonic acid supported on carriers (British Patent No. 1,017,806 and Japanese Patent Kokoku Nos. 42-5222 and 43-20286).
The process of carrying out esterification in liquid phase using acid catalysts such as sulfuric acid and phosphoric acid is generally carried out. However, since esterification of organic carboxylic acids is an equilibrium reaction, it is essential for increasing equilibrium conversion to remove and discharge the by-produced water from reaction system. Thus, the process suffer from many economical problems that alcohol must be used in excess amount, complicated means such as azeotropy with addition of other solvents are required and besides, corrosion of apparatuses is conspicuous.
The process of carrying out esterification in liquid phase using acidic cation exchange resins as catalysts has the merit that separation of reaction mixture and catalyst is easy and form of reaction is simplified, but has the economical disadvantages that cost of the resin is relatively high and reduction in activity due to deterioration of heat resistance and duration of life is unavoidable and regeneration or exchange at intervals is necessary.
Furthermore, when unsaturated carboxylic acids such as acrylic acid, methacrylic acid and crotonic acid are used as the organic carboxylic acids in these liquid phase processes, since starting materials and products are polymerizable, there occur troubles that efficient operation is hindered or recovery of product is lowered due to deposition of polymer in reaction apparatus or connecting lines.
The process of esterification in gas phase can solve the various problems in the process of esterification in liquid phase because the polymerization of unsaturated carboxylic acid does not substantially take place in gas phase state and furthermore, equilibrium shifts to the direction favorable for production system "AIChE Journal", 14(4), 606 ('68) ].
However, there are still significant problems of insufficient activity and life of catalysts and the process has not yet been industrialized.
The object of the present invention is to provide an industrially advantageous process for producing organic carboxylic acid esters which is free from the above-mentioned defects in conventional processes.
The inventors have made intensive research in an attempt to find an excellent process for producing organic carboxylic acid esters in gas phase.
As a result, it has been found that reduction of activity of catalyst which supports sulfuric acid in esterification in gas phase is caused by desorption of the supported sulfuric acid and when sulfuric acid in an amount corresponding to the amount of sulfuric acid desorbed is continuously or intermittently supplied, no reduction of catalyst activity occurs and moreover, very high esterification activity can also be stably obtained by similarly supplying a catalytic amount of sulfuric acid in the presence of only carrier. Thus, the present invention has been accomplished.